seismic design

(Thanks to my new Moleskine, I was able to scribble a note to myself about this interview at a stoplight on the way home from school Friday)

This a portion of “Designing a Bridge for Earthquake Country”, an NPR Science Friday Interview with Dr. Marwan Nader, lead design engineer of the new span of California’s Bay Bridge.


In the interview, Nader describes that the Bay bridge is built on a foundation of soil, not rock, which amplifies seismic motion – a problem in California. The new Bay Bridge has not just been designed, but seismically designed as a self-anchored suspension bridge, which differs from the ground anchorage of a typical suspension bridge. Nader explains that instead the cable is anchored to the deck of the bridge, so the loads the cable bears go to the deck. Another difference is that the cables are three-dimensional, instead of the vertical cables on typical suspension bridges, which satisfies the need for design equilibrium.

(images from Wikipedia: self-anchored suspension bridge) The above image is of a traditional suspension bridge. Note that the anchors are in ground.

Versus this self-anchored suspension design in which the cables are anchored directly to the deck.


FLATOW: …also, I noticed from the design is that the bridge is made in difference pieces so that parts can move independently of one another, correct?

NADER: That’s correct.

FLATOW: Doing so, so that when the Earth shakes, it all just sort of floats.

NADER: Right. The seismic design, the way we understand it, is basically there are effectively two ways to resist the motions. One is to really design a bunker, which effectively is very strong to take the forces…

FLATOW: So you’re fighting nature.

NADER: Yes. And what you’re doing there, is you’re really taking on whatever the motions are. And the earthquake has a very interesting characteristic to it. It’s like a musical, effectively. It’s got areas where there’s a lot of energy, which is at the frequencies that are very, very low or very, you know, very, very high. Excuse me. And then that’s where you’re getting the most energy. And then as you get the structure to be more flexible, that’s where the energy gets smaller. So if you are a little bit careful about it, you can actually design your structure to be in the areas where the earthquake is less damaging. And by making that structure tuned to what Mother Nature’s going to apply, you actually avoid that ground of the force.

The other aspect of it is designing components, if you will, that are made to take on the damage. Like when we drive cars. If you think about it, cars – we know we drive cars. We know that we’d like not to get into accident, but we planned for that accident. And the idea…

FLATOW: It’s like crumple zones.

NADER: Exactly. And the idea behind it, is you get the damage to occur in areas where you keep the car functional to the extent possible when it’s a midsized type of accident that you have. And the idea is that the fenders take all the damage. Very similar to that, is our bridge is designed that way. We actually looked at specific areas which we said that’s where it makes sense to have the damage occur. We designed those elements to take on that damage, and thereby protecting the more important elements to it.

FLATOW: So you can replace those damaged pieces later on.

NADER: Exactly. The idea is that, after an event, the bridge is still functional. We would go in – obviously, the engineers at that time would go in and do a, you know, an inspection, evaluate – there will be damage, but it will be in a form where you can actually make it available so that emergency traffic can be – immediately after that, go on it, and shortly after that go through normal traffic.

The bridge

Please don’t read this as writing is the bridge to ______________; I’m more interested in the design of the bridge itself. I don’t know much about the engineering of bridges, but this idea really struck me as something interesting for the design of composition, and as something that fit within/alongside my ideas of page tectonics: seismic design. Here is a new metaphor for conceptual use. Designs that are functionally flexible, that can withstand shifts and even damage when the larger body (composition) moves. Where would such a design allow us to go that we couldn’t reach before?